JP-A-10-48543 shown below describes a method of driving a microelectromechanical device array of DMD (digital micromirror device) or the like in a background art. The drive method of the background art will be explained in reference to FIGS. 9 to 11.
FIG. 9 is a constitution view of two devices of a microelectromechanical device array. A semiconductor board 1 is formed with a drive circuit, not illustrated, at inside thereof, and a surface portion of the semiconductor board 1 is formed with movable mirrors 2, 3.
The respective movable mirrors 2, 3 each is supported above a hollow space by a hinge 6 hung between supports 4, 5 respectively erected on the surface of the semiconductor board 1 and is made to be pivotable in a left and right direction centering on the hinge 6. The hinge 6 is integrally formed with electrode films 7, 8 in the left and right direction by interposing the hinge 6, and the surface of the semiconductor board 1 is formed with fixed electrode films 9, 10 at positions of being opposed to the electrode films 7, 8.
When a bias voltage Vb=24 V is applied to the hinge 6 (electrode films 7, 8) of the movable mirror 2 as a control voltage, an address voltage Va=5 is applied to the fixed electrode film 9, and an address voltage Va=0 is applied to the fixed electrode film 10 respectively as device-displacing signals, a voltage difference DV between the electrode films 7, 9 becomes DV=19 V, a voltage difference DV between the electrode films 8, 9 becomes DV=24 V, and by a difference between an electrostatic force between the electrode films 7, 9 and an electrostatic force between the electrode films 8, 10, the movable mirror 2 is inclined in a direction of bringing the electrode films 8, 10 into contact with each other. An illustrated state shows a state of inclining the movable mirror 2 by −10°.
Similarly, when the bias voltage Vb=24 V is applied to the hinge 6 (electrode films 7, 8) of the movable mirror 3, an address voltage Va=9 is applied to the fixed electrode film 9, and an address voltage Va is applied to the fixed electrode film 10, a voltage difference DV between the electrode films 7, 9 becomes DV=24 V, a voltage difference DV between the electrode films 8, 10 becomes DV=19 V, and by a difference between an electrostatic force between the electrode films 7, 9 and an electrostatic force between the electrode films 8, 10, the movable mirror 3 is inclined in a direction of bringing the electrode films 7, 9 into contact with each other. An illustrated state shows a state of inclining the movable mirror 3 by plus 10°.
When incident light is irradiated to the movable mirrors 2, 3, directions of reflected light differ in accordance with inclinations of the movable mirrors 2, 3, and by controlling the inclinations of the movable mirrors 2, 3, directions of reflected light can be controlled to ON/OFF.
However, it is difficult to operate a plurality of mirrors in the same direction or in an inverse direction independently from one another and simultaneously with one another and therefore, in the background art, the movable mirrors are controlled to be driven by carrying out a complicated voltage control. The control will be explained in reference to FIGS. 10 and 11.
The inclined movable mirror 2 is shown at a topmost stage of FIG. 10. When the movable mirror 2 inclined to a left side is changed to a following state, there are two ways of “following state”. That is, there are a case of inclining the movable mirror 2 to an opposed side (right side) and a case of inclining the movable mirror 2 to the same side (left side) (case of maintaining an inclined state). To which state the movable mirror 2 is changed depends on image data to be formed when the microelectromechanical device array is used as an image forming apparatus.
A drawing on a left side surrounded by a frame at a lower stage of FIG. 10 shows a case of displacing the movable mirror 2 to an opposed side (Crossover transition) and a drawing on a right side shows a case of maintaining an inclined state of the movable mirror 2 as it is (Stay transition). An address voltage Va applied to the fixed electrode films 9, 10 of the respective movable mirrors 2, 3 is controlled respectively for the movable mirrors 2, 3, and a bias voltage Vb is commonly applied to all of the movable mirrors.
When the state of inclining the movable mirror is made to transit to a following state, the bias voltage Vb is changed as shown by FIG. 1. When a time period from starting to change to finish to change the movable mirror is divided to zones A, B, C, D, E, first, at zone A, the bias voltage is set to Vb=24 V, at zone B, set to Vb=−26 V. At next zone C, the bias voltage is set to Vb=7.5 V, at zone D, the bias voltage is returned to Vb=24 V, and at zone E, the bias voltage is maintained at Vb=24 V.
At zone A, the address voltage Va is applied (or rewritten) to 0 V or 5 V. In changing the movable mirror to a following state, when the movable mirror is intended to be inclined by making the electrode films 7, 8 moved integrally with the movable mirror proximate to the fixed electrode film 9, the voltage Va applied to the fixed electrode film 9 is set to 0 V, and when the movable mirror is intended to be inclined by making the electrode films 7, 8 proximate to the fixed electrode film 10, the voltage Va applied to the fixed electrode film 10 is set to 0 V and the voltage Va applied to the opposed side electrode film is set to 5 V. Therefore, the address voltage Va is also referred to as an device-displacing voltage (or an device-displacing signal).
When the applied voltage is controlled in this way, as shown by a left side (crossover side) of FIG. 10, at zone B, the bias voltage becomes Vb=−26 V, the voltage difference DV between the electrode films 8, 10 becomes DV=33.5 V and the voltage difference DV becomes DV=26 V between the electrode films 7, 9. Thereby, the movable mirror 2 is applied with an electrostatic force of further inclining the movable mirror 2 to the left side, and the electrode film 8 is further pressed to the fixed electrode film 10 in a state of being brought into contact with the fixed electrode film 10 to be elastically deformed. Further, although the state is described as “contact” for convenience of explanation, actually, a gap is maintained between the two electrode films, and the electrode films are not electrically shortcircuited.
At next zone C, when the bias voltage becomes Vb=7.5 V, the voltage applied to the fixed electrode film 10 is set to Va=7.5 V. Thereby, the voltage difference between the electrode films 8, 10 becomes DV=0 and the voltage difference between the electrode films 7, 9 becomes DV=7.5 V. Thereby, an electrostatic force is generated between the electrode films 7, 9, a repulsive force by elastically deforming the electrode film 8 at zone B is added to the electrostatic force to detach the electrode film 8 from the electrode film 10, and the movable mirror 2 starts rotating in the clockwise direction.
At next zone D, when the bias voltage becomes Vb=24 V, the difference between the electrode films 8, 10 becomes DV=16.5 V, the voltage difference between the electrode films 7, 9 becomes DV=24 V, the electrostatic force operated between the electrode films 7, 9 is further intensified, and the movable mirror 2 is rotated further in the clockwise direction.
At final zone E, the electrode film 7 of the movable mirror 2 is impacted to the address electrode film 9. At this occasion, the voltage applied to the address electrode film 10 is set to Va=5 V. The movable mirror 2 is slightly vibrated as shown by FIG. 11 by the impact and thereafter brought into a stable state to finish the operation of inclining to the opposed side.
In order to bring the movable mirror 2 in a state on a right side (stay side) of FIG. 10, as shown by an upper stage on a right side in a frame of FIG. 10, the voltage applied to the fixed electrode film 10 is set to Va=0 (zone A). At next zone B, when the bias voltage becomes Vb=−26 V, the voltage applied to the fixed electrode film 9 on the opposed side is set to Vb=7.5 V and at next zone C, the bias voltage becomes Vb=7.5 V.
At this occasion, as shown by a dotted line circle H in FIG. 11, the electrode film 8 is temporarily detached from the electrode film 10, at zone D, when the bias voltage becomes Vb=24 V, the electrode film 8 is brought into contact with the electrode film 10 again, thereafter, at zone E, the voltage applied to the electrode film 9 is set to Va=5 V, and a state of inclining the movable mirror 2 is maintained to a state of being inclined to a left side.
According to the above-described method of driving the microelectromechanical device array of the background art, at zone C, when the movable mirror of the crossover side device (device for changing from left inclination to right inclination or changing from right inclination to left inclination) is inclined to the opposed side, at zone B, the electrode film 8 is temporarily pressed to the side of the fixed electrode film 10 and the electrode film 8 is detached from the fixed electrode film 10 by also utilizing the repulsive force. At this occasion, also according to the movable mirror on the stay side, at zone C, the electrode film 8 is temporarily detached from the fixed electrode film 10, the above-described repulsive force is not utilized.
That is, according to the driving method of the background art, by a difference in a speed of detaching the electrode film 8 from the fixed electrode film 10 (whether the repulsive force is utilized), the movable mirrors on the crossover side and on the stay side are separated. Therefore, there is brought about an erroneously operated movable mirror unless the difference in the detaching speed can accurately be controlled. Further, the bias voltage Vb is changed from +24 V to −26 V and therefore, a burden applied on the drive circuit is considerable.